Linkage between isozyme markers and a locus affecting seed size in Phaseolus vulgaris L.

Backcross and F2 progenies were produced between two bean genotypes, 'XR-235' and 'Calima,' which differ in seed weight by a factor of two. The small-seeded 'XR-235' was used as the pistillate and recurrent parent. These genotypes showed polymorphisms at nine isozyme loci and at the phaseolin locus. Seed size parameters (weight, length, width, and thickness) were determined for each BC1 and F2 individual, i.e., for seeds harvested from 'XR-235' after pollination with F1 and from the F1 after selfing, respectively. A combination of starch gel electrophoresis and enzyme activity staining was used to determine the genotype of each BC1 and F2 individual at the segregating loci. SDS-PAGE and Coomassie blue staining were used to determine geno-type at the phaseolin locus. Tests for independent assortment using two-way contingency and maximum likelihood tables revealed three linkage pairs: Aco-1 - 20 cM - Dia-1; Adh-1 - 2 cM - Got-2; and Est-2 - 11 cM - Pha. Statistical comparisons were made between the means of genotype classes at each segregating locus for all seed size parameters. The results from two independently obtained BC1s and the F2 consistently indicated that the Adh-1-Got-2 segment was linked to a locus that affected seed size and overcame maternal control over seed size. This locus has been designated Ssz-1. This gene exhibited additive gene action and accounted for 30-50% of the seed size difference between the parents.